Air seeders and other seeding devices are commonly towed by tractors to apply seed or fertilizer, or both simultaneously, to a field. As an example, an air seeder may be towed in combination with a tilling implement, one behind the other, to place the seed and fertilizer under the surface of the soil. An air seeder has as its central component a wheeled seed cart which comprises one or more frame-mounted product tanks for holding product, generally seed or fertilizer or both. Air seeders also generally comprise a metering system for dispensing product from the tanks and a pneumatic distribution system for delivering the product from tank to soil.
It is known to provide an air seeder with product tanks which are pressurized with air from the pneumatic distribution system. The pneumatic distribution system of an air seeder generally utilizes a centrifugal fan to provide at least one airstream which flows through the pneumatic distribution system to seed boots where product is deposited in the soil. Product is first introduced to the air stream by the metering system at venturi in a primary distribution manifold located below the metering system. Product is carried by the air stream through distribution lines to a series of secondary distribution manifolds ("headers"), which in turn distribute product through distribution lines to seed boots mounted behind ground openers on the tilling implement so that the product may be evenly delivered to the span of ground acted upon by the tilling implement (the "tillage"). In positive pressure seeding systems, some air is diverted from the pneumatic distribution system to the product tanks to pressurize the tanks. Ideally, the tanks are maintained at the same pressure as the airstream which carries product from the product tank.
Conventional air seeders convey pressurized air to the tank by taking air from a plenum immediately downstream from the fan. The air pressure at the plenum may differ from the air pressure in the primary distribution manifold, where product is introduced to the pneumatic distribution system.
Operators of conventional seeding equipment have problems maintaining a correct level of air pressure in the product tanks. If air pressure in the product tanks is too low the product will not enter the airstream correctly and pulsing will occur. If air pressure is too high, "blow-by" of unmetered product by the bottom or top of the meter will occur. Either of these situations is undesirable.
Therefore, it is desirable to devise a system for taking air pressure from the airstream that product is metered into in order to maintain the proper pressure differential between the product tank and a venturi in the primary distribution manifold.